If you are on a dedicated server look at serverconfig.xml
<property name="MaxSpawnedZombies" value="64" /> <!-- This setting covers the entire map. There can only be this many zombies on the entire map at one time. Changing this setting has a huge impact on performance. -->
If hosting co-op on your PC look at registry key HKEY_CURRENT_USER\Software\The Fun Pimps\7 Days To Die\MaxSpawnedZombies_h1091629070
Those values are total zombies on the whole server, not per person.